The present invention relates to a screen printing apparatus which prints paste such as solder cream or conductive paste on a substrate.
In an electronic parts mounting process, as methods of printing paste such as solder cream or conductive paste on a substrate, screen printing has been used. In this method, a mask plate in which pattern holes are provided according to portions of printing is set on the substrate, and the paste is printed on the substrate through the pattern holes of the mask plate by squeezing.
As squeezing methods of this screen printing, a method using a closed-type squeeze head has been known (for example, refer to PCT WO00/48837). This method is different from the usual screen printing in that the paste is not supplied directly on the mask plate but a squeeze head provided with a paste storing container is used. In this method, in a state where an opening provided in the lower surface of the paste storing container is brought into contact with the mask plate, the paste in the paste storing container is pressurized thereby to be pushed into the pattern hole of the mask plate through the opening. By sliding the squeeze head on the mask plate, the paste is applied into each pattern hole in order.
In the screen printing using the above closed-type squeeze head, the paste in the squeeze head is always in a state where it is shut off from the outside. Accordingly, in a process of repeating the squeezing operation, the temperature of the paste in the squeeze head tends to increase. Therefore, the temperature of the paste increases with passage of time, and its viscosity changes, so that there is a problem that appropriate rolling flow is obstructed in squeezing and bad printing is easy to be produced.